κ-Bungarotoxin

κ-Bungarotoxin was first reported in 1983 as a component of the venom of Bungarus multicinctus that differed in biological effect from the previously known α-bungarotoxin: κ-bungarotoxin, but not α-bungarotoxin, was capable of impeding nicotinic signaling in the chick ciliary ganglion.[4] Bungarotoxin toxin was designated "kappa" as an allusion to the Latin word kiliaris ("from the eye"), and to the root of "ciliary".[4] Separately identified toxins designated "toxin F" and "bungarotoxin 3.1" were identified by protein sequencing as identical to κ-bungarotoxin.[2]

κ-Bungarotoxin binds to the nicotinic acetylcholine receptors of the autonomic ganglia, predominantly to the nicotinic receptor subunit alpha 3 (CHRNA3) and to a lesser extent alpha 4. Two distinct binding surfaces, both on the N-terminal extracellular face of the receptor subunit, have been identified.[5]

κ-Bungarotoxin is a receptor antagonist, meaning it blocks the normal response of the receptor to acetylcholine, which inhibits neurotransmission and therefore causes neuromuscular paralysis. Like the α-bungarotoxins, κ-bungarotoxin causes a post-synaptic blockade of signaling; this is in contrast to the β-bungarotoxins which induce a pre-synaptic block.[3] The distinction between the effects of α-bungarotoxin and κ-bungarotoxin was first identified functionally, as differences in effects on specific neural structures.[4][6] The basis of this functional difference has been molecularly characterized as differences in receptor subtype specificity; the pentameric receptors are assembled from different distributions of subunits in neurons and in muscles.[5]

The κ-bungarotoxin polypeptide is 66 amino acids long and folds into an antiparallel beta sheet structure stabilized by five conserved disulfide bonds, a structural feature shared by many peptide toxins. Unlike other members of the bungarotoxin family, κ-bungarotoxin is a dimer.[1]